This invention relates to blow molding and particularly to equalizing the length of successive hollow thermoplastic parison portions consecutively received between closing sections of a series of adjacent blow molds in a continuously cycling system.
In blow molding hollow articles such as containers by closing sections of a partible mold on successive lengths of a hollow parison issuing from an annular orifice in an extruder head, a waste "tail" portion is formed which must be removed before the article can be considered to be in finished form. Such tail represents the portion of the tubular parison squeezed into a web configuration between the mold sections during closing which extends beyond the cavity defining the surface configuration of the hollow article, and is well known in the art as necessary primarily to ensure the presence of sufficient material to form the end wall of the article being blow in the mold.
The length of such tail protruding beyond the cavity in a continuously operating blow molding system will vary from mold to mold for a variety of reasons. For example, successive lots of polymer may have slightly different rheological properties which can result in different melt flow behavior in the extrusion system. Changes in the ratio of recycled regrind to virgin material can also produce rheological variations in the resultant mixture. When programming the wall thickness of the extruding parison to optimize distribution in the subsequently blown container, different patterns of flow of the plastic between the parison-forming surfaces will occur if the movable die surface of the orifice is not repositioned exactly in successive cycles. As the maximum capacity of the extrusion system supplying the molding machine is approached, (such condition being desirable to minimize molded part cost in a high volume system) surge of the output at the orifice can occur which magnifies variability.
Such variability in tail length can cause serious processing problems. For example, if the mold is closed without forming any tail at all, the free end of the parison will not be pinched shut and the too short length cannot be blown at all. If a tail is formed but is real short, downstream tail removal equipment set to accommodate a particular range of lengths may fail to pick it up and the article can go through the system untrimmed. On the other hand, if the tail is too long, the mold sections may be unable to close tightly on each other, or preset downstream tail-removal and article handling equipment may jam, either of such conditions undesirably requiring shut down of the overall production line. When employing thickness programming as previously described, as tail length varies the program pattern will wander in the direction of variance and therefore optimum wall distribution will not be attainable during subsequent blowing.
U.S. Pat. No. 3,759,648 discloses a system for controlling tail length which involves changing the speed of rotation of the extruder screw in accordance with whether the sensed extruded length of parison below the outlet of the extruder head is too long or too short. When screw speed is varied, however, the rheological properties of the thermoplastic polymer in the extruder are also changed. For example, increasing or decreasing rotary or axial movement of the screw will correspondingly cause the temperature and therefore the viscosity of the polymer melt to go up or down because of the change in energy input. As a consequence, such a control system tends to be constantly hunting to correct its own introduction of instability.